A sense of adventure seeking out OP dinos during an event - say, Halloween- aside from DodoRex and DodoWyvern - mini-bosses you have to put some actual work and anticipation into finding, even if they're variants of regular creatures. An all-in Halloween would be absolutely amazing! Some examples of this for, air, land, and sea - though could come up with more for other events, Halloween is just of course the next big one:
Alpha Ravager Pack (Chupacabra) - Find clues on the dead Ovis left on the map to find the next spawn location and time of the Chupacabra pack, who summon undead Ravagers as minions!
Enraged Squid (Kraken) - Come on, everyone wants one of these kind of boss battles. Beyond Alpha, make a true sea boss battle, deep, deep sea with a Kraken, Leedsicthys to destroy boats above, and other squid to fight in an epic underwater battle. Find the sunken ship locations to discover where and when to find the Kraken next!
Air - honestly Dodowyverns are still great and can't think of much better.
And just for fun ideas, more because they'd be funny or annoying
Enraged Kairuku - These penguins are PISSED. After days of beating them and their babies for their organic polymer, they aren't having it any more - Kairuku's are now Enraged (glowing eyes of course), and their strength is on par to a turkey. Good luck getting that polymer!
Undead Icthyornis & Pegos - 50% more spawns - because honestly those birds from hell scare me more than a Rex.